1. Field of the Invention
The invention relates to semiconductor integrated circuits, and more particularly to a semiconductor integrated circuit containing an oscillator and reducing Electro-Magnetic interference (hereinafter, abbreviated to EMI) noise produced by the oscillator.
2. Description of the Prior Art
As is well known, smaller sets of electrical equipment are introduced because of the recent technical trend of module integration that is proceeding in the automotive electrical equipment industry. This leads to an increasing demand for reducing EMI noise in FM band/keyless band (Rf band) of the individual semiconductor integrated circuits (semiconductor devices) installed in the set, which is also well known.
It is also well known that a variety of factors are responsible for EMI noise of such semiconductor integrated circuits.
EMI noise caused by an oscillator integrated in the above described semiconductor integrated circuit is always emitted in almost all the operating modes thereof except when the oscillator is in a standby mode to be stopped. Therefore, the reduction of noise caused by the oscillator is one of the challenges for the reduction of EMI noise.
A semiconductor integrated circuit containing such a conventional oscillator is disclosed, for example, in Japanese Patent Laid-Open No. 02-228106.
FIG. 1 is a block diagram of a semiconductor integrated circuit containing a conventional oscillator. In a semiconductor integrated circuit 900 containing a conventional oscillator, an output A1 from an oscillating section 910 is input to a waveform shaping section 920 including a Schmitt circuit S8, and an output signal 925 is output via an output buffer BUF 5.
The Schmitt circuit S8 is inserted for the purpose of blocking noise superimposed on output from the oscillating section 910, and it serves to increase Electro-Magnetic susceptibility (hereinafter, abbreviated to EMS) characteristic. In addition to this, specifically in the course of oscillation growth from the start of oscillation, the Schmitt circuit serves to reduce EMI noise caused by through-current by eliminating changes in its state in the case in which the output A1 from the key oscillating section 910 is at the intermediate potential.
FIG. 2 shows the results of Fast Fourier transform (hereinafter, abbreviated to FFT) analyses of an oscillation power-source current Ivddosc flowing through the oscillation circuit and a waveform-shaping section power-source current Ivdd flowing through the waveform shaping section including the Schmitt circuit when the semiconductor integrated circuit containing the conventional oscillator shown in FIG. 1 is in stable oscillation.
Also, FIG. 3 shows current waveforms at that time. In the conventional oscillator, the oscillation power-source current Ivddosc flowing through the oscillation circuit in stable oscillation has a peak at the oscillation frequency of a point P, and the spectra fall downward with increasing frequency.
This is because a current flowing through a feedback inverter within the oscillating section changes smoothly as shown in the oscillation power-source current Ivddosc of FIG. 3, due to the fact that the output A1 of the key oscillating section exhibits a waveform close to a sine wave, wherein the current flowing through the feedback inverter is caused by through-current and charging current into capacitance including internal and external parasitic capacitance. In contrast to this, although the waveform-shaping section power-source current Ivdd has a lower spectrum in the vicinity of the point P, its spectra does not fall in strength to the high frequency region, exhibiting a spectrum distribution that is nearly white.
In the Schmitt circuit S8, transistor element P13 and N13 are constantly switched according to feedback signals, and their through-current also complicatedly change with changes in gm of the transistor elements. Because a signal with a rectangular waveform shaped by the Schmitt circuit S8 is input to the output buffer (BUF5) connected to the following stage, the through-current of the buffer becomes steep. Therefore, the waveform-shaping section power-source current Ivdd flowing through the waveform shaping section becomes steep and complicated, as shown in the waveform-shaping section power-source current Ivdd of FIG. 3, and thus its spectra spreads to the high frequency region.
Also, semiconductor integrated circuits containing the conventional oscillator as described above are further disclosed in Japanese Patent Laid-Open No. 02-174248 and Japanese Patent Laid-Open No. 11-145727.
However, the conventional prior art has a problem that the insertion of the Schmitt circuit contrary increases EMI noise due to the following reasons.
In other words, there is a problem in which the insertion of the Schmitt circuit deteriorates EMI characteristics in the high frequency region because of an increased through-current induced by the adding of transistor element itself and the complicated current characteristics of the Schmitt circuit.
Further, as measures against EMI, a typical, known method is to insert a pass capacitor near a noise source. However, in the case of the oscillator, there is also a problem that insertion of a pass capacitor in the vicinity of the oscillator, is not so effective for the reduction of noise produced by the waveform shaping section including the Schmitt circuit since the capacitor is mostly used for charging and discharging of the feedback inverter within the oscillating section, as shown for the oscillation power-source current Ivddosc in FIG. 3.